Sunflower
Sunflower (Helianthus annuus L.), is one of the major annual crop species grown for edible oil. Others include soybean (Glycine max L.), peanut (Arachis hypogaea L.), rape or canola (Brassica spp.), and cotton (Gossypium spp.). The oil extracted from sunflower seeds is highly regarded by consumers in the United States due to its bland taste, cooking qualities, and fatty acid profile. See Heiser, C. B., 1978, Taxonomy of Helianthus and Origin of Domesticated Sunflower, IN Sunflower Science and Technology, American Society of Agronomy, Madison, Wis.
A dicot, sunflower is a member of the family Compositae and usually bears seeds on a single terminal head. It is distinguished from all other cultivated plants by its single stem and large inflorescence. See Knowles, P. F., 1978, Morphology and Anatomy, IN Sunflower Science and Technology. American Society of Agronomy, Madison, Wisconsin. Sunflower and another member of its genus, Jerusalem artichoke (H. tuberosus L.), are also distinct in that they are the only important food plants domesticated in prehistoric times in the portion of North America which has become the United States.
In the United States, hybrid sunflower seeds are typically produced by seed companies and sold to farmers. On farms, hybrid sunflowers are usually grown as a row crop. During the growing season herbicides are widely used to control weeds; fertilizers are used to maximize yields; and fungicides and insecticides are used to control disease pathogens and insect pests. At maturity in the fall, sunflower seeds are usually harvested with a combine. From farms, harvested sunflower seeds are transported to crushing plants, where the edible oil is extracted therefrom. The edible oils are then sold to consumers, used in preparing food products, or serve as raw materials for other industrial uses.
While the agronomic performance of sunflower hybrids has improved, there is always a need to develop better hybrids with increased seed and oil yields. Moreover, heat and drought stresses and continually changing insect predators and disease pathogens present hazards to farmers as they grow sunflower hybrids. Thus, there is a continual need for sunflower hybrid varieties which offer higher seed yields and oil percentages in the presence of heat, drought, pathogens, and insects.
Inbred Lines and Hybrid Varieties
The ultimate purpose for developing sunflower inbred lines is to be able to dependably give rise to hybrids. Commercially viable sunflower hybrids, like hybrids in many other crop species, manifest heterosis or hybrid vigor for one or more economically important traits.
Plants resulting from inbreeding, usually self-pollination (selfing), for several generations are termed inbred lines (inbreds). These inbreds are homozygous at almost all gene loci. When selfed, these inbreds produce a genetically uniform population of true breeding inbred progeny. These inbred progeny possess genotypes and phenotypes essentially identical to that of their inbred parent. A cross between two different inbreds produces a genetically uniform population of hybrid F.sub.1 plants which are heterozygous for many gene loci. By contrast, a cross of two plants, which are not inbreds and are themselves heterozygous at a number of gene loci, will produce a population of progeny which are heterozygous at many loci but which are not genetically uniform.
The significance of this phenomenon is two-fold. First, seed supplies of inbreds may be maintained by selfing the inbreds. Equivalently, seed supplies of inbreds may be increased by growing inbred plants in isolation such that only pollen from that inbred genotype is available for fertilization during flowering (anthesis). Second, because the inbred lines themselves are genetically uniform, hybrids from inbred parents always have the same appearance and uniformity and can be produced by crossing the same set of inbred lines whenever desired. Thus, a hybrid created by crossing a defined set of inbreds will always be the same. Moreover, once the inbreds which give rise to a superior hybrid have been identified, a continual supply of the hybrid seed can be produced using these inbred parents.
Pollen Control
Commercial sunflower hybrids are typically produced by crossing two inbred lines. In order to effect such a cross the pollen-producing portion of the inflorescence from one of the inbred lines must either be removed or otherwise rendered sterile. Sunflower inflorescences are perfect and thus possess both stamens and pistils therein. Thus, hand or mechanical emasculation is not economically feasible for commercial hybrid production. However, several options for controlling male fertility are available. These options include cytoplasmic-genetic male sterility, genetic male sterility, and gametocides.
Hybrid sunflower seed is typically produced by a cytoplasmic-genetic male sterility system (CMS). This system requires both a homozygous nuclear locus and a cytoplasmic factor for sterility. Otherwise, the plant will produce viable pollen. The CMS requires A-lines (females), B-lines (maintainers), and R-lines (males). A-lines are homozygous for a nuclear allele for pollen sterility and possess a cytoplasmic factor for pollen sterility as well. A-lines are thus male-sterile. B-lines are homozygous for the sterile nuclear allele, but possess a fertile cytoplasmic factor. B-lines produce viable pollen. Moreover, B-lines usually have a nuclear genome essentially identical to a complimentary A-line. R-lines are homozygous for a fertile nuclear allele and possess a fertile cytoplasmic factor. Thus, R-lines produce viable pollen. Seed of A-lines is increased by pollinating A-line plants with pollen from complimentary B-lines. The resulting seed from A-lines pollinated with pollen from B-lines is also male-sterile because the fertile cytoplasmic factor from B-lines is not transmitted by pollen. Hybrid seed is produced by pollinating A-line plants with pollen from R-line plants. The resulting hybrid seeds are heterozygous at the nuclear locus and possesses the sterile cytoplasmic factor. Thus, the hybrid seed will grow into plants which produce viable pollen.
In commercial hybrid seed production, alternate strips of a female and a male inbred variety are planted in a field. Provided that there is sufficient isolation from sources of foreign pollen, inflorescences of the female inbred (A-line) will be fertilized only by pollen from the male inbred (R-line). If so, the resulting seed is exclusively a single hybrid. Often a hive of bees is placed near the production field to ensure that sufficient bees are present to effect pollination.
Plant Breeding
The use of male-sterile inbreds is but one factor in the production of sunflower hybrids. The development of sunflower hybrids also requires the development of homozygous inbred lines, the crossing of these lines to form hybrid seed, and the agronomic evaluation of the hybrids. Thus, breeding programs combine the genetic backgrounds from two or more inbred lines or various other broad-based germplasm sources into breeding pools from which new inbred lines are developed by selfing and selection for desired phenotypes. The newly developed inbreds are crossed with other inbred lines. The hybrids from these crosses are evaluated to determine which have commercial potential.
Thus, the invention of a new hybrid sunflower variety involves a number of steps. These steps broadly include:
(1) selecting plants from germplasm pools for initial breeding crosses; PA1 (2) crossing the selected plants in a mating scheme to effect breeding crosses; PA1 (3) selfing and selecting progeny from the breeding crosses for several generations to produce a series of newly developed inbred lines, which, although different from each other, breed true and are highly uniform; PA1 (4) crossing the newly developed inbred lines with other unrelated inbred lines to produce hybrid seeds; and PA1 (5) evaluating the hybrids in performance trials to determine their value as new commercial varieties. PA1 1. Substantial phenotype. The phenotype of hybrid sunflower PAN 9612 as herein described, including minor modifications and variations which do not affect the agronomic performance or the end use properties thereof and referring to the hybrid sunflower seed, hybrid sunflower plant germinating and growing from the hybrid sunflower seed, and all tissues therefrom. PA1 2. Regenerable tissue. Tissue arising from the hybrid sunflower seed or the hybrid sunflower plant designated and described herein as PAN 9612 which is capable of being regenerated directly into plants or of being cultured into callus, the callus then being regenerated into plants. PA1 3. Embryogenesis. The process of initiation and development of bipolar plant structures from either zygotic cells, somatic cells, or callus cells. PA1 4. Callus. Undifferentiated tissue usually cultured in vitro on a synthetic medium. PA1 5. Inbred, inbred line, or inbred parent. A relatively true breeding strain resulting from successive generations of inbreeding, such as self-pollination or from successive generations of backcrossing to a recurrent parent until the phenotype of the recurrent parent is substantially recovered and the trait from the donor parent is present. PA1 6. Hybrid or hybrid variety. First-generation (F.sub.1) progenies from a cross between two inbred lines, between two hybrids, or between a hybrid and an inbred line. PA1 1. SEED YIELD (or YIELD) (LBS./A). Seed yield in pounds per acre adjusted to a basis of 10-percent moisture. PA1 2. PCT. OIL (10%). Percent oil of harvested seeds, adjusted to a basis of 10 percent moisture. PA1 3. OIL YIELD (LBS/A). Pounds of oil per acre. Calculated by multiplying seed yield by the percent oil then dividing by 100. PA1 4. HEIGHT (IN). Average plant height (in). The average distance from the base of the stem to the base of the inflorescence (head or capitulum) of a genotype. PA1 5. DAYS TO FLOWER-MID. The average length of time in days from emergence until fifty percent of the plants of a genotype begin anthesis. Anthesis begins when the outer whorl of disk flowers have opened. PA1 5a. DAYS TO FLOWER, FIRST. The average length of time in days from emergence until the first plant of a genotype begins anthesis. PA1 6. SEED MOISTURE (%). The average percent moisture of the harvested seed for a genotype. PA1 7. LODGING (%). The average percentage of plants at harvest time in which the stems are tilting from the vertical at an angle of more than fifteen degrees. PA1 8. TEST WEIGHT (LBS/BU). Test weight in pounds per bushel. The bulk density of the harvested seeds (achenes) of a genotype. PA1 9. NECK BREAK (%). The percentage of plants at harvest time with broken necks, a neck being the intermode extending between the head and the nearest stem node basal to the head. PA1 1. Class. Oil or non-oil type (confectionery). PA1 2. Maturity PA1 3. Height PA1 4. Stem PA1 5. Leaves (midstem at flowering) PA1 6. Head (at flowering) PA1 7. Head (at seed maturity) PA1 8. Seeds. PA1 9. Disease Reactions. PA1 10. Insect Reactions
During the inbreeding process, the vigor of the lines typically decreases to some extent. However, vigor is restored when two different inbred lines are crossed to produce the hybrid progeny.
Aside from the complexities of the choices in the above steps, there are many important factors to be considered in the art of plant breeding. These factors include the breeder's ability to recognize important morphological and physiological characteristics, the ability to design evaluation techniques for genotypic and phenotypic traits of interest, and the ability to search out and exploit the genes for the desired traits in new or improved combinations.
The objective of commercial sunflower hybrid development programs is thus to develop new inbred lines. These new lines are then used to produce hybrids which, in turn, produce high seed and oil yields in the presence of environmental hazards. The primary trait sunflower breeders seek to improve is either total seed yield or total oil yield. However, many other major agronomic traits are of importance in hybrid combination and have an impact on yield. Such traits include percent moisture at harvest, percent oil in harvested seeds, plant height, test weight, days to flower, days to maturity, resistance to stalk breakage, resistance to lodging, seed quality, and resistance or tolerance to temperature and moisture stress and to disease pathogens and insects.
The inbred lines per se must also have acceptable performance levels for parental traits such as seed yields, seed sizes, and pollen production. All of these traits affect the ability of a parental line to produce seed in economically sufficient quantities. Many if not all of these traits are affected by several genes.
Pedigree Breeding
The pedigree method of breeding is a widely used methodology for new line development. Generally this procedure involves crossing two or more inbred parent lines to produce an F.sub.1 generation, then self-pollination of the F.sub.1 generation to produce the F.sub.2 generation. The F.sub.2 generation and subsequent progeny segregate for all traits in which the inbred parent lines differ. An example of the process in which an F.sub.2 generation is present is set forth below. Variations of this generalized pedigree method are used. However, all variations produce a segregating generation which contains a range of variation for the traits in which the inbred parents differ.
Hypothetical Example of Pedigree Breeding Program
Consider a cross between two inbred lines which differ for alleles at six loci. The parental genotypes are:
______________________________________ Parent 1 AbCdeF/AbCdeF Parent 2 aBcDEf/aBcDEf The F.sub.1 from a cross between these two parents has the genotype: F.sub.1 AbCdeF/aBcDEf Selfing the F.sub.1 will produce an F.sub.2 generation which includes the following genotypes: AbcDEf/abCdeF AbcDef/abCdEF AbcDef/abCdeF . . . ______________________________________
The number of possible genotypes in the F.sub.2 generation is 3.sup.6 (=729). But the F.sub.2 generation will produce only (2.sup.6)-2 (=62) possible new inbreds. However, only a very limited proportion of these combinations will be useful. Thus, only a very small proportion of the progeny from F.sub.2 individuals can give rise to progeny possessing these new and useful allelic combinations.
It has been shown that many traits of economic value in sunflowers are under the genetic control of multiple genetic loci, and that there are a large number of unique combinations of these genes present in sunflower germplasm. Fick, G. N., 1978, Breeding and Genetics, IN Sunflower Science and Technology, American Society of Agronomy, Madison, Wis.
By way of example, if one assumes the number of segregating loci in an F.sub.2 generation to be somewhere between 20 and 50 and one further assumes that each parent is fixed for half of the favorable alleles present, it is then possible to calculate approximate probabilities of producing an inbred which has a favorable allele at {(n/2)+m} loci, where n/2 is the number of favorable alleles in each of the parents and m is the number of additional favorable alleles in the new inbred. See Probability of Developing an Inbred With m of n Favorable Alleles, below. The number m is assumed to be greater than three because each allele has such a small effect that evaluation techniques are not sensitive enough to detect differences due to three or fewer favorable alleles. The probabilities in the example below are the probabilities that at least one genotype with (n/2)+m favorable alleles will occur.
Probability of Developing an Inbred With m of n Favorable Alleles
Assume each parent has n/2 of the favorable alleles and only 1/2 of the combinations of loci are economically useful.
______________________________________ No. of No. of No. of favorable favorable alleles segregating alleles in Parents in new inbred Probability that loci (n) (n/2) (n/2) + m genotype occurs* ______________________________________ 20 10 14 3 .times. 10.sup.-5 24 12 16 2 .times. 10.sup.-5 28 14 18 1 .times. 10.sup.-5 32 16 20 8 .times. 10.sup.-6 36 18 22 5 .times. 10.sup.-6 40 20 24 3 .times. 10.sup.-6 44 22 26 2 .times. 10.sup.-6 48 24 28 1 .times. 10.sup.-6 ______________________________________ *Probability that a useful combination exists does not include the probability of identifying this combination if it does occur.
As can be seen from above, these probabilities are on the order of 10.sup.-5 or smaller. The probability of being able to identify this improved combination (genotype) based on replicated field testing would most likely be smaller than these values. The probability of being able to identify an improved genotype by replicated performance trials is a function of the population size of genotypes tested and how testing resources a re allocated in the testing program.